1. Field of the Invention
The present invention relates to a thermosensitive recording material, and more particularly to an improved thermosensitive recording material comprising a support, an intermediate layer comprising an expanded plastic filler with minute voids prepared by expanding an expandable plastic filler, formed on the support, and a thermosensitive coloring layer formed on the intermediate layer, with an electroconductive agent being contained in at least one of the intermediate layer or the thermosensitive coloring layer.
2. Discussion of Background
It is conventionally known that a colorless or light-colored coloring leuco dye reacts with a color developer such as an organic acidic material to be colored upon application of heat thereto when the leuco dye and the color developer are fused. Thermosensitive recording materials utilizing this coloring reaction are conventionally known as are disclosed, for instance, in Japanese Patent Publication Nos. 43-4160 and 45-14039.
Recently, these thermosensitive recording sheets have been employed in a variety of fields, for instance, for use with recorders for measurement instruments, terminal printers for computers, facsimile apparatus, automatic ticket vending apparatus, and bar code readers.
In accordance with recent remarkable improvements in the application of the above-mentioned recording apparatus to a variety of new fields and in the performance thereof, there is a great demand for thermosensitive recording sheets which can complement those improved apparatus. More specifically, there are demanded thermosensitive recording sheets capable of yielding sharp images with high density with application of a small amount of thermal energy with the increase in the thermal head speed. In addition, thermosensitive recording sheets are required to satisfy the sufficient thermal head matching properties such as not forming any dust from the recording material in the course of thermosensitive recording by use of the thermal head which comes into contact with the surface of the thermosensitive recording material and not sticking to the thermal head.
Color development on the thermosensitive recording sheet can be attained by the melting of a coloring leuco dye or a color developer or both of them and the successive reaction of the two to induce a color in the leuco dye under the application of thermal energy from the thermal head.
As a method of increasing the thermal coloring sensitivity of the thermosensitive recording material, there is widely known a method of adding to the thermosensitive recording material a thermofusible material which melts at a temperature lower than the melting points of the employed leuco dye and the color developer and is capable of dissolving herein the leuco dye and color developer when melted.
A variety of such thermofusible materials for use in this method has been proposed, for example, nitrogen-containing compounds such as acetamide, stearamide, m-nitroaniline, and phthalic acid dinitrile in Japanese Laid-Open Patent Application No. 49-34842; acetoacetic anilide in Japanese Laid-Open Patent Application No. 52-106746; and alkylated biphenyl alkanes in Japanese Laid-Open Patent No. Application 53-39139.
However, in accordance with the recent development of a high speed thermal facsimile apparatus, it has become a common practice to energize and drive a thermal head at high speed. Under such circumstances, there is the problem that the background of a thermosensitive recording sheet is also colored due to the build-up of thermal energy in the thermal head and therearound in the course of repetition of high speed thermosensitive recording. In order to solve this problem, it is one of the most important subjects to be cleared to enhance the dynamic thermal coloring sensitivity of the conventional thermosensitive recording material, without decreasing the coloring initiation temperature thereof. In the conventional thermosensitive recording materials the dynamic thermal coloring sensitivity cannot be increased unless a large amount of the above-mentioned thermofusible compounds is added, although the static thermal coloring sensitivity can be increased to some extent by the addition of the thermofusible compounds. However, when a large amount of the thermofusible materials is employed, the sticking and dust-adhesion problems occur in the course of thermal recording by a thermal head. Further, when the melting points of the employed thermofusible compounds are extremely low, the preservability of the thermosensitive recording sheets is significantly degraded so that the fogging of the background of the recording material occurs during the storage thereof.
For the purpose of increasing the dynamic thermal coloring sensitivity of the recording material, there have been proposed a method of smoothing the surface of the thermosensitive coloring layer, and a method of increasing the concentration of the coloring components by decreasing the relative amounts of components which do not contribute to the coloring reaction of the thermosensitive recording material, such as fillers and binder agents.
The method of smoothing the surface of the thermosensitive coloring layer is usually carried out without difficulty by use of a supercalender. However, this method has the shortcomings that the background of the thermosensitive coloring layer is colored during the calendering process and the surface of the thermosensitive coloring layer becomes so glossy that the appearance of the recording material is impaired.
Fillers such as calcium carbonate, clay, and urea-formaldehyde resin may be added to the thermosensitive coloring layer to maintain the whiteness of the background of the coloring layer and to prevent the sticking and dust adhesion problems during the thermosensitive recording using a thermal head. Further a water-soluble binder agent may be added to the thermosensitive coloring layer to firmly bind the coloring components and other additives of the thermosensitive coloring layer to the support. When the amounts of such a filler and a water-soluble binder agent are reduced, the desired properties for the thermosensitive recording material cannot be obtained.
In addition, there have been proposed thermosensitive recording materials, in which an intermediate layer with a sufficient heat insulating properties, including a foamed portion with minute voids, is interposed between a support and a thermosensitive coloring layer. For example, a thermosensitive recording material in which an intermediate layer comprising as the main component microballoon particles is interposed between a support and a thermosensitive coloring layer is proposed in Japanese Laid-Open Patent Application No. 59-5093; and a thermosensitive recording material comprising a support, an intermediate layer A comprising an expanded plastic filler prepared by expanding an expandable plastic filler formed on the support, an intermediate layer B comprising a filler and a binder agent formed on the intermediate layer B, and a thermosensitive coloring layer formed on the intermediate layer B is proposed in Japanese Laid-Open Patent Application No. 59-225987.
To prepare the intermediate layer including the expanded plastic filler with minute voids, a method of coating on the support a liquid comprising an expandable plastic filler in an unexpanded state and then expanding the expandable plastic filler under application of heat thereto is generally used. However, this method has the shortcomings that the surface of the obtained intermediate layer becomes uneven after heated, and the obtained images lack surface uniformity.
As a solution to the above-mentioned problem, in the thermosensitive recording material as disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 59-225987, the intermediate layer A which includes the expanded filler is covered with the intermediate layer B, so that the surface of the intermediate layer A is made smooth. Alternatively, a method of subjecting the intermediate layer to calendering can be used to improve the surface smoothness thereof.
However, in spite of the above-mentioned countermeasure, it has become obvious that such intermediate layers including expanded plastic fillers with minute voids have a serious problem with respect to static electricity when industrially mass-produced. More specifically, the expandable plastic filler, in the expanded state, contained in the intermediate layer not only has high electrical resistance by itself, but also can be easily electrostatically charged by slight friction because the water content in the intermediate layer is significantly reduced in the course of heating process. Therefore, during the manufacturing operation of a large amount of the thermosensitive recording sheets comprising an intermediate layer including the expanded plastic filler with minute voids, the intermediate layer is disadvantageously attached to the surface of the transportation rollers due to its electrostatic charge, especially when a liquid for the formation of the intermediate layer is coated on the support and the intermediate layer is subjected to calendering. Further, when these semi-fabricated recording sheets in which the intermediate layer is formed on the support are wound around a winder or unwound off an unwinder, sparking is apt to occur due to the electrical discharge of the recording sheets, so that it is quite difficult to manufacture high-quality products steadily and safely.
Even though the conventional thermosensitive recording sheets are completed by coating the thermosensitive coloring layer on the above-mentioned intermediate layer, they are easily electrostatically charged, so that they cannot always be smoothly carried in the thermal printing apparatus for practical use.